Various sensor elements for determining the concentration of a gas component in gas mixtures are available. Thus, for example, so-called lambda sensors are used to determine the excess-air coefficient, which describes the ratio of air to fuel. This ratio is of decisive significance for the combustion of the fuel in an internal combustion engine and for the exhaust gas aftertreatment.
Conventional discrete-level sensors and broadband sensors may measure the residual oxygen content in the exhaust gas. In a discrete-level sensor, the potential of an exhaust-side electrode is measured in relation to an oxygen-flushed reference electrode. A discrete-level sensor may recognize the transition from a rich mixture to a lean mixture and vice versa. A broadband lambda sensor may measure the residual oxygen content in the exhaust gas over a substantially wider range, i.e., both in the rich range and also in the lean range. It generally includes a combination of a typical concentration sensor (Nernst sensor) acting as a galvanic cell and a limiting current or “pump” cell. A voltage is externally applied to the pump cell. If the voltage is sufficiently high, a limiting current results, which is proportional to the difference of the oxygen concentration on both sides of the cell. Oxygen atoms are transported with the current as a function of the polarity. Precisely enough oxygen from the exhaust gas is always supplied to the concentration sensor from the pump cell by an electronic control circuit so that the state λ=1 prevails. The particular pump current, which is proportional to the oxygen content or rich gas content in the exhaust gas, forms the output signal of the broadband lambda sensor. The measurement of the concentration in the measuring gas chamber is performed on the basis of the determination of the Nernst voltage between a Nernst electrode in the measuring gas chamber and an oxygen-flushed reference electrode in a reference chamber. In order to reach the operating temperature required for the oxygen ion transport, a broadband lambda sensor is equipped with an integrated heating device.
The measuring signal of the lambda sensor is a function of both the excess-air coefficient λ, i.e., the ratio of air to fuel in the mixture, and also the prevailing absolute pressure. The absolute pressure of the exhaust gas varies by several hundred millibars with the frequency of the cylinder ignition of the internal combustion engine. With each pressure pulse, the quantity of lean or rich gas components transported into the measuring gas chamber of the lambda sensor is briefly strongly increased and subsequently strongly decreased again. Since the controller of the pump current reacts very rapidly, it can pump out rapidly in the phase of transporting in the additional gas components, which represent a deviation from the setpoint concentration in the cavity of λ=1. Therefore, fewer gas components are transported out in the subsequent low-pressure phase than were transported in during the high-pressure phase. The occurring oscillations of the pump current may be smoothed by a suitable electronic filter. Overall, however, a shift of the mean pump current occurs, which is accompanied by a loss of characteristic curve precision.
An approach for minimizing the inaccuracy accompanying this uses the densest possible embodiment of the diffusion barriers, which separate the exhaust gas from the measuring gas chamber. However, the static pressure dependence of the lambda sensor is increased in this way. Another measure for reducing the effects of the dynamic pressure dependence is to shrink the volume of the measuring gas chamber in comparison to the volume of the diffusion barrier, as described in German Patent Application No. DE 10 2004 023 004 A1, for example. The dependence of the measuring signal on the dynamic pressure variations may also be decreased in this way. However, shrinking the measuring gas chamber may also result in other disadvantages. Due to the high diffusion resistance of the cavity, the oxygen is only pumped out at the front edge of the electrode in this case and thus the electrode is locally overloaded. This is true in particular for poisoning by gaseous electrode poisons transported thereto.
The present invention relates to an example method for operating a sensor element, in particular a broadband lambda sensor, which allows reliable compensation of the dynamic pressure dependence of the pump current and therefore increases the measuring precision of the sensor element. The example method is also to be able to be used in existing sensor elements, without further modifications having to be made on the sensor element itself.